In this paper, for the first time, we investigated the combined effect of subsurface dams and a typical stratified aquifer (two high-permeability layers with a low-permeability layer between them) on groundwater flow and salinity distribution in a tidally influenced coastal unconfined aquifer. Subsurface dams can inhibit the invasion of saltwater, and the low-permeability layer (LPL) and tide action can increase the effect of subsurface dams and the removal rate of residual saltwater. Through sensitivity analysis, it was discovered that shifting the dam location towards the inland resulted in a reduction in the effective heights of the dam. The upper saline plume contracted with increasing dam height, and the upper boundary of LPL was moved to shallower regions. And the natural removal time increased significantly with increasing dam height and the bottom boundary of LPL was moved to deeper regions. In addition, if the dam location was close to the sea boundary and the bottom boundary of LPL was moved to deeper regions, we could increase the subsurface dam height to reduce the risks of control of saltwater intrusion. This study provides us a comprehensive understanding of the complex hydrodynamics of saltwater intrusion and provides guides for the design of subsurface dams aimed at saltwater intrusion control in stratified coastal aquifers.

Radon is recognized as a powerful tracer of certain geophysical processes in marine and aquatic environments. In the past few decades, the instruments and methods for measuring radon concentration in water have been developed to some extent but still lack underwater in-situ measurements. Here we present an in-situ detection equipment for radon-in-water (pulsed ionization chamber (PIC)-radon) to measure dissolved radon in ocean and groundwater settings. The equipment has been successfully deployed in the Jiaozhou Bay in July 2022 and has achieved 14 d of unattended underwater in-situ observation. Then it was successfully placed in a groundwater monitoring well in the Laizhou Bay in November 2022 and monitored radon activities for over 30 d. The results showed that this instrument had a good indication of submarine groundwater discharge. The PIC-radon detector takes advantage of smaller size, lower power consumption, and is barely influenced by humidity, making it particularly suitable for long-term in-situ measurement, especially in harsh environments with limited human care or deployment spaces.

A high-frequency, high-resolution shore-based video monitoring system (VMS) was installed on a macrotidal (tidal amplitude >4 m) beach with multiple cusps along the Quanzhou coast, China. Herein, we propose a video imagery-based method that is coupled with waterline and water level observations to reconstruct the terrain of the intertidal zone over one tidal cycle. Furthermore, the beach cusp system (BCS) was precisely processed and embedded into the digital elevation model (DEM) to more effectively express the microrelief and detailed characteristics of the intertidal zone. During a field experiment conducted in January 2022, the reconstructed DEM was deemed satisfactory. The DEM was verified by RTK-GPS and had an average vertical root mean square error along corresponding RTK-GPS-derived intertidal profiles and corresponding BCS points of 0.134 m and 0.065 m, respectively. The results suggest that VMSs are an effective tool for investigating coastal geomorphic processes.

The cross-shore variation in wind speeds influenced by beach nourishment, especially the dramatic changes at the nourished berm, is important for understanding the aeolian sand transport processes that occur after beach nourishment, which will contribute to better beach nourishment project design on windy coasts. In this paper, the influencing factors and potential mechanism of wind speed variation at the edge of a nourished berm were studied. Field observations, together with the Duna model, were used to study the cross-shore wind speed distribution for different nourishment schemes. The results show that the nourished berm elevation and beachface slope are the main factors controlling the increase in wind speed at the berm edge. When the upper beach slope is constant, the wind speed at the berm edge has a positive linear correlation with the berm elevation. When the berm elevation remains constant, the wind speed at the berm edge is also proportional to the upper beach slope. Considering the coupling effects of nourished berm elevation and beachface slope, a model for predicting the wind speed amplification rate at the nourished berm edge was established, and the underlying coupling mechanism was illustrated.

Low tide terrace beach is a main beach type along South China coasts with strong tidal actions. How strong tides affect wave transformations on low tide terrace beach still remains unclear. In this study, in-situ measurements are conducted on the low terrace beach at Xisha Bay to provide quantitative descriptions of wave shoaling and shore-breaker phenomena under the tidal effects. It is found that wave breaking is unsaturated on the low tide terrace beach at Xisha Bay. Magnitudes of wave skewness and asymmetry increase as wave shoals and achieve the maximum value at the shore-breaker, and then decrease rapidly. Mean energy dissipation rates of shore-breakers are tide-modulated since the bottom slope changes at the shoreward boundary of wave propagation in a tidal cycle. The remaining wave energy flux at the initialization of the shore-breaker is 1%–12% of offshore wave energy flux, and the energy flux ratio decreases with increasing offshore wave heights. Wave attenuation at shore-breakers can be estimated directly from offshore wave conditions based on findings in this study, favoring designs of seawalls or beach nourishment projects. Field datasets on wave transformations can also be used for verifications of wave numerical models.

Fences have been widely used in coastal protection engineering for their low cost, simple deployment, and easy integration with ecosystems. The effects of fence porosity and height on dune development have been investigated while not much attention has been paid to the effects of fence opening configurations, such as opening size and geometry, and porosity distributions. In this study, we deployed eight fences with same height and similar porosity, but different opening configurations on a sandy beach in Pingtan, Fujian Province. Results indicate that there is a similar two-dune-one-trough pattern for all fences at the beginning of dune development, and opening size, orientation, and geometry, and porosity distribution control the leeward dune peak locations. Fences with small openings and non-uniform porosity have high trapping efficiency, and upper denser porosity may be the optimal design as these fences have the highest trapping efficiency and capacity. The conclusions from this study can provide guidance on practical fence design.

Mangrove forest is one of the most important ecological and environmental resources by effectively promoting tidal flat deposition and preventing the coastal region from typhoon. However, there have been mass loss of mangrove forests due to anthropogenic activities. It is an urgent need to explore an effective way for mangrove restoration. Here, three rows of bamboo fences with hydro-sedimentary observation set over Aegiceras corniculatum mangrove tidal flat of the Nanliu Delta, the largest delta of Beibu Gulf, China, were conducted to analyze the hydro-sedimentary variations induced by bamboo fences. Results identified that the mean horizontal velocity U_m per burst (20 min) decreased by as much as 71% and 40% in comparison with those without bamboo fences in March and November, respectively, when the tidal current entering the bamboo area during flood. The maximum of mean horizontal flow velocity U_m-max at bamboo area was 50%–75% of that without bamboo fences during ebb tide. The suspended sediment concentration of bamboo area suggested a maximum reduction of 57% relative to bare flat during flood, and was 80% lower than bare flat at ebb peak. Moreover, the turbulent kinetic dissipation ε at flood tide was significantly higher than that at ebb tide, while the bamboo fences greatly increased the turbulent kinetic dissipation ε by 2 to 5 times relative to bare flat, resulting in an increase of the bed elevation by inhibiting the sediment incipient motion and intercepting suspended sediment. The siltation rate at the bamboo area was 140% and 29.3% higher than that at the bare flat and the region covered with A. corniculatum, respectively. These results highlight that bamboo fences can effectively attenuate tidal current and thus promote siltation over mangrove flat, which contribute great benefit to mangrove survival.

Beach erosion has occurred globally in recent decades due to frequent and severe storms. Dongsha beach, located in Zhujiajian Island, Zhejiang Province, China, is a typical embayed sandy beach. This study focused on the morphodynamic response of Dongsha beach to typhoon events, based on beach topographies and surficial sediment characteristics acquired before and after four typhoon events with varying intensities. The four typhoons had different effects on the topography and sediment characteristics of Dongsha beach. Typhoons Ampil and Danas caused the largest (−51.72 m³/m) and the smallest erosion (−8.01 m³/m), respectively. Remarkable alongshore patterns of beach profile volumetric changes were found after the four typhoon events, with more erosion in the southern and central parts of the beach and few changes in the northern part. Grain size coarsening and poor sorting were the main sediment patterns on the beach influenced by different typhoons. Typhoons that occurred in the same year after another typhoon enhanced the effect of the previous typhoon on sediment coarsening and sorting variability, but this cumulative effect was not found between typhoons that occurred during different years. A comparison of the collected data revealed that the topographic state of the beach before the typhoon, typhoon characteristics, and tidal conditions were possible reasons for the difference in the responses of Dongsha beach to typhoon events. More severe beach erosion was caused by typhoons with higher intensity levels and longer durations, and high tide levels during typhoons can determine the upper limit of the beach profile erosion site. Taken together, these results can be used to improve beach management for storm prevention.

Based on the measured beach profile data of Sanzhou Bay from 2015 to 2019, an erosion hotspot was identified along the Shanwei coastline of eastern Guangdong, where the maximum retreat distance of the shoreline exceeded 80 m and the erosion rate was more than 20 m/a. To determine the time at which the erosion hotspot started and the potential causes of its formation, this study used 63 Landsat satellite images from 1986 to 2019 to construct a time series of shoreline positions over the past 30 years by extracting their high-tide shorelines. Next, the M-K trend test method was introduced to evaluate the non-linear shoreline behavior based on the single-transect method. The results showed that the time of approximately 2013 marked the start of the erosion hotspot, the erosion hotspot was characterized by erosion rates of more than 2 m/a (a maximum rate of 31.6 m/a), and the affected shoreline more than 4.3 km from 2013 to 2019. Furthermore, this erosion hotspot was proved to be caused by artificial sand mining in the nearshore zone, which destroyed the original beach’s morphodynamic equilibrium. With the aid of storm events, soil cliffs composed of loose sediment on the backshore were sacrificed to achieve a new equilibrium, resulting in an extremely significant retreat parallel to the coast on the west side of the study area, which reflects the combined effect of human and natural processes. This study provides a concrete example of the rapid response of shorelines to artificial sand mining activities, and the associated finding is a stark warning about the cautious development and utilization of coastal zones and the strict regulation of human activities.

To explore the nourishment effect and disaster reduction efficiency of a fully dissipative dry beach under the impact of storms, this paper uses the measured topography and hydrodynamic data to establish a one-dimensional numerical model of the XBeach beach profile. By numerically modeling the change in the nourished profile for different dry beach widths under normal waves and storm conditions and the recovery process of the profile after the storm, the degree of response in dry beach nourishment for the fully dissipative beach is analyzed. The results show that under normal wave conditions, the response of the nourished dry beach is obvious. Sediment on the dry beach erodes heavily, and the shoreline moves landward over a long distance. With the increase in the width and size of the dry beach, the wave height at the bottom of the backshore profile decreases, the wave height attenuation rate increases continuously, and the wave elimination effect is remarkable. When the storm incident wave intensifies, the wave height attenuation rate of the nourished dry beach decreases, indicating that the smaller the storm intensity is, the more significant the wave reduction effect of the nourished dry beach is. At the same time, different profile arrangements of nourished dry beaches suffer from different degrees of erosion under storm conditions, with significant changes in profile morphology. With intensified storm action, the intensity of sediment erosion in the nourished dry beach increases, the nourishment is weakened, and the recovery effect of the profile after the storm is not obvious. The results of the numerical modeling highlight that the dry beach nourishment method can resist storms to a certain extent, but the overall effect is relatively limited.